JP6642558B2 - Premixed compression ignition engine - Google Patents

Description

  The present invention relates to a premixed compression ignition engine capable of performing premixed compression ignition combustion in which fuel including gasoline is self-ignited while being mixed with air, and assisting ignition of the air / fuel mixture by a forced ignition source.

  In an engine that performs homogeneous charge compression ignition combustion (for example, see Patent Literature 1), as the load of the engine increases and the engine speed increases, the combustion pressure rises sharply and the combustion period becomes excessively short (abrupt combustion). Heat generation). When this premature combustion occurs, problems such as an increase in combustion noise and abnormal combustion occur. In order to prevent this problem, it is desirable to make the combustion period moderately long by burning relatively slowly in the early stage of combustion and relatively quickly in the late stage of combustion, thereby forming a late-centroid heat generation pattern.

  Combination of SI (Spark Ignition) combustion and CI (Compression Ignition) combustion is effective in obtaining the late-centroid heat generation pattern. That is, a mixture in which the air-fuel mixture in the combustion chamber is forcibly ignited to perform combustion by flame propagation (SI combustion) and the unburned air-fuel mixture in the combustion chamber is self-ignited by the heat generated by the SI combustion (CI combustion). It is a combustion method.

JP 2013-194712 A

  In the above-described combined combustion system, flame propagation in SI combustion may not uniformly spread in all directions of the combustion chamber due to in-cylinder flow such as tumble flow. In this case, the region where self-ignition based on CI combustion occurs in the combustion chamber space due to the uneven distribution of the flame propagation increases, and the amount of heat generated by the self-ignition becomes excessive, and the intended combustion noise reduction effect and abnormal combustion suppression effect Is not obtained.

  An object of the present invention is to reduce combustion noise and suppress abnormal combustion by preventing as much as possible the uneven distribution of flame propagation in a combustion chamber in a homogeneous charge compression ignition engine that performs ignition assistance.

A premixed compression ignition engine according to one aspect of the present invention is an ignition assisted premixed compression ignition engine that uses at least fuel containing gasoline, and includes an intake port and an exhaust port, and intake air from the intake port. A tumble flow is generated from the intake port side toward the exhaust port side, and a combustion chamber for compression ignition combustion of the air-fuel mixture generated by the supply of the fuel, and the combustion chamber is directed toward a radially central region of the combustion chamber. A fuel injection valve for injecting fuel, a forced ignition source having an ignition unit disposed in the combustion chamber , and an ignition control unit for controlling ignition timing by the forced ignition source, to partition the combustion chamber A part of the combustion chamber wall is formed by a crown surface of the piston and a combustion chamber ceiling surface facing the crown surface, and the intake port and the exhaust port are formed by the combustion chamber ceiling. The crown surface is formed of a cavity recessed in a central region in the radial direction, and a plane arranged at least radially outside the cavity on the intake side and substantially parallel to the ceiling surface of the combustion chamber. A squish generating surface, wherein the ignition portion is located eccentrically on the intake side when the side where the intake port is arranged in the combustion chamber is the intake side and the side where the exhaust port is arranged is the exhaust side. Wherein the fuel injection valve is disposed near the inside of the outer peripheral edge of the cavity, the fuel injection valve injects fuel into the cavity in a middle stage of a compression stroke, and the ignition control unit performs compression after injection of the fuel. a timing before the top dead center, the combustion chamber ceiling surface than an extended line the ignition portion in sectional view along the cylinder axis direction is extended line along the squish generated surface radially inward And the tumble flow flowing along the cavity and the squish flow flowing along the squish generation surface merge and the in-cylinder flow toward the radial center region of the combustion chamber is ignited. The forced ignition source is controlled such that the mixture is forcibly ignited at a timing formed at the position of the section .

  According to this homogeneous charge compression ignition engine, even if a mixture is formed in the radial center region of the combustion chamber by the fuel injection of the fuel injection valve, the flame generated by the forced ignition is directed to the exhaust side by the influence of the tumble flow. Flow, and the flame propagation tends to be biased toward the exhaust side. However, the ignition portion of the forced ignition source is arranged unevenly on the intake side. Thereby, the position deviated to the intake side in the combustion chamber becomes the forced ignition point, and the flame propagation can be directed to the radially central region of the combustion chamber on the tumble flow from the intake side to the exhaust side. As a result, the combustion due to the flame propagation is prevented from being biased toward the exhaust side, so that the flame propagation combustion occurs exclusively in the radial center region of the combustion chamber, and then the self-ignition combustion occurs in the peripheral region of the combustion chamber. Therefore, the amount of heat generated by self-ignition does not become excessive, and it is possible to reduce combustion noise and suppress abnormal combustion.

  In the above-mentioned homogeneous charge compression ignition type engine, a part of a combustion chamber wall surface that partitions the combustion chamber is formed by a piston crown surface and a combustion chamber ceiling surface facing the crown surface, and the intake port and the combustion port The exhaust port is open to the combustion chamber ceiling surface, and the crown surface is provided with a cavity recessed in a radially central region and at least a radially outside of the cavity on the intake side in the combustion chamber. A squish generation surface formed of a plane substantially parallel to the ceiling surface, wherein the ignition section merges the tumble flow flowing along the cavity and the squish flow flowing along the squish generation surface in a later stage of the compression stroke. It is desirable to be arranged in the confluence area where it joins. The term “substantially parallel” includes not only a mode in which the combustion chamber ceiling surface and the squish generation surface are parallel to each other, but also a mode in which one has an inclination of about ± 5 degrees or less with respect to the other. Meaning.

  According to this homogeneous charge compression ignition engine, it is possible to form an air-fuel mixture in the cavity. Then, by arranging the ignition portion in the merging area, the tumble flow and the squish flow from the squish generation surface toward the radial center of the combustion chamber are used to transfer the flame propagation to the radial center of the cavity. Can be directed to the part. In the initial stage of the expansion stroke, self-ignition combustion in the radially outer region of the combustion chamber can be promoted by the reverse squish flow generated on the squish generation surface.

  In the above-described homogeneous charge compression ignition engine, the cavity includes a bottom and a slope rising from the bottom toward the outer peripheral edge of the cavity, and the squish generation surface is formed from the outer peripheral edge of the cavity to the crown surface. A plane extending radially outward, wherein the merging region is radially inward of the outer peripheral edge portion where the squish generation surface extends in a cross-sectional view along the cylinder axis direction, It is desirable that the area be adjacent to the upper part of the slope.

  Since the above-mentioned area is an area where the tumble flow and the squish flow merge naturally, it is optimal as a location where the ignition part is arranged.

  In the above-mentioned homogeneous charge compression ignition type engine, the intake port and the exhaust port are each opened two by two on the ceiling surface of the combustion chamber, and the squish generation surface is provided between the two intake ports. A plane extending radially outwardly of the crown surface, wherein the confluence region is inside the outer peripheral edge of the cavity in a plan view in the cylinder axial direction of the combustion chamber, and the two intake ports It is desirable to be an area between the two.

  The above-mentioned region is a region where the tumble flow and the squish flow merge in a combustion chamber employing a two-valve intake / exhaust configuration (four-valve type). Therefore, in such a combustion chamber, the location of the ignition portion is optimal.

  In the above-described homogeneous charge compression ignition engine, in the fuel injection from the fuel injection valve, an air-fuel mixture is formed in the cavity, and an air layer is formed between the air-fuel mixture and a wall surface of the cavity. It is desirable to be performed as follows.

  According to this premixed compression ignition engine, the gas mixture layer and the air layer surrounding the gas mixture layer are stratified in the cavity, and the cooling loss can be reduced. Then, in such a state that the fuel distribution is stratified, the flame propagation can be uniformly generated in all directions in the cavity.

  ADVANTAGE OF THE INVENTION According to this invention, in the homogeneous charge compression ignition type engine which performs ignition assistance, reduction of combustion noise and suppression of abnormal combustion can be aimed at by preventing uneven distribution of flame propagation in a combustion chamber as much as possible.

FIG. 1 is a schematic diagram of a configuration of an engine system to which a homogeneous charge compression ignition engine according to the present invention is applied. FIG. 2 is a schematic sectional view along the cylinder axis direction of the engine body. FIG. 3 is a block diagram showing a control configuration of the homogeneous charge compression ignition engine. FIG. 4 is a time chart showing the relationship between fuel injection, forced ignition, and heat generation. FIG. 5A is a cross-sectional view illustrating a state of formation of an air-fuel mixture in a combustion chamber, and FIG. 5B is a plan view illustrating a mode of combustion in the combustion chamber. FIG. 6 is a plan view of the crown face of the piston and is a view for explaining the layout of the ignition portion. FIG. 7 is a cross-sectional view along the cylinder axis direction of the combustion chamber and is a diagram for explaining a layout of an ignition portion. FIGS. 8A and 8B are schematic cross-sectional views along the cylinder axis direction of the engine main body for describing a tumble flow and a squish flow generated in the combustion chamber. FIGS. 9A and 9B are diagrams illustrating a flame propagation zone generated in the comparative example. FIGS. 10A and 10B are diagrams showing a flame propagation zone generated in the present embodiment. FIG. 11 is a diagram illustrating a state in a later stage of combustion in the present embodiment. FIG. 12 is a graph showing heat generation rates in the combustion chambers of the comparative example and the present embodiment.

[Overall configuration of engine]
Hereinafter, a homogeneous charge compression ignition engine according to an embodiment of the present invention will be described in detail with reference to the drawings. FIG. 1 is a schematic configuration diagram of an engine system to which a homogeneous charge compression ignition engine according to an embodiment of the present invention is applied. The engine system according to the present embodiment includes a four-stroke engine body 1, an intake passage 20 for introducing combustion air into the engine body 1, and an exhaust passage 30 for discharging exhaust generated by the engine body 1. And an EGR device 40 that recirculates part of the exhaust gas passing through the exhaust passage 30 to the intake passage 20 as EGR gas.

  FIG. 2 is a schematic cross-sectional view of the engine body 1 along the cylinder axis direction A. The engine body 1 is an in-line four-cylinder engine in which four cylinders 2 are arranged in series in a direction orthogonal to the plane of the paper of FIG. FIG. 2 shows only one of the four cylinders 2. The engine system is mounted on a vehicle, and the engine body 1 is used as a drive source of the vehicle. In the present embodiment, the engine body 1 is a premixed compression ignition engine driven by receiving a supply of fuel including gasoline. The fuel may be gasoline containing bioethanol or the like.

  The engine body 1 includes a cylinder block 3, a cylinder head 4, and a piston 5. The cylinder block 3 has a cylinder liner that forms the above-described four cylinders. The cylinder head 4 is attached to an upper surface of the cylinder block 3 and closes an upper opening of the cylinder 2. The piston 5 is accommodated in each of the cylinders 2 so as to be slidable back and forth, and is connected to the crankshaft 7 via a connecting rod 8. In response to the reciprocating motion of the piston 5, the crankshaft 7 rotates around its central axis.

  A combustion chamber 6 is formed above the piston 5. An intake port 9 and an exhaust port 10 communicating with the combustion chamber 6 are formed in the cylinder head 4. The bottom surface of the cylinder head 4 is a combustion chamber ceiling surface 6U, and the combustion chamber ceiling surface 6U has a pent roof shape having a slightly upwardly inclined surface. In the ceiling surface 6U of the combustion chamber, an intake side opening which is a downstream end of the intake port 9 and an exhaust side opening which is an upstream end of the exhaust port 10 are formed. An intake valve 11 for opening and closing the intake side opening and an exhaust valve 12 for opening and closing the exhaust side opening are assembled to the cylinder head 4.

  The intake valve 11 and the exhaust valve 12 are so-called poppet valves. The intake valve 11 includes an umbrella-shaped valve element that opens and closes the opening of the intake port 9 and a stem that extends vertically from the valve element. Similarly, the exhaust valve 12 includes an umbrella-shaped valve element that opens and closes the opening of the exhaust port 10 and a stem that extends vertically from the valve element. Each of the valve bodies of the intake valve 11 and the exhaust valve 12 has a valve surface facing the combustion chamber 6. The engine body 1 is a double overhead camshaft type (DOHC) engine (see FIG. 6). The intake side opening and the exhaust side opening are provided two by two for each cylinder 2, and the intake valve 11 and the exhaust Two valves 12 are also provided.

  The bottom surface of the combustion chamber 6 is defined by a crown surface 50 of the piston 5. A cavity 5 </ b> C is provided in the crown surface 50. The cavity 5 </ b> C has a substantially circular shape in a top view, and is formed to be recessed downward in a central region of the crown surface 50 in the radial direction B of the combustion chamber 6. A squish generation surface 51 is provided outside the cavity 5C in the radial direction B. The squish generation surface 51 is formed of a plane parallel to the pent roof type combustion chamber ceiling surface 6U, and the gap therebetween is substantially constant in the radial direction B. In addition, the surface of the crown 50 is coated with the heat insulating layer 5H.

  In the present embodiment, the combustion chamber wall surface that partitions the combustion chamber 6 includes an inner wall surface of the cylinder 2, a crown surface 50 of the piston 5, a combustion chamber ceiling surface 6 </ b> U opposed to the crown surface 50, an intake valve 11 and an exhaust valve 12. Of each valve surface. The geometric compression ratio of the engine body 1 of the present embodiment, that is, the volume of the combustion chamber 6 when the piston 5 is at the bottom dead center and the volume of the combustion chamber 6 when the piston 5 is at the top dead center. Is set to a high compression ratio of 13 or more and 35 or less (for example, about 20).

  An ignition plug 13 (forced ignition source) for forcibly igniting the air-fuel mixture in the combustion chamber 6 is mounted on the cylinder head 4, one for each cylinder 2. The ignition plug 13 has an ignition portion 13A provided with an electrode that discharges a spark and gives ignition energy to the air-fuel mixture. The ignition plug 13 is attached to the cylinder head 4 in such a manner that the ignition portion 13A projects into the combustion chamber 6. In the present embodiment, when the side where the intake port 9 is disposed in the combustion chamber 6 is the intake side and the side where the exhaust port 10 is disposed in the combustion chamber 6 is the exhaust side, the ignition portion 13A is unevenly disposed on the intake side. Is characterized by This will be described in detail later.

  An injector 14 (fuel injection valve) for injecting fuel into the combustion chamber 6 is mounted on the cylinder head 4 (combustion chamber ceiling surface 6U), one for each cylinder 2. A fuel supply pipe (not shown) is connected to the injector 14, and the injector 14 is an external valve-type injector that injects fuel supplied through the fuel supply pipe into the combustion chamber 6, and is provided at a lower end of the injector 14. Is provided with a head portion 14A having a fuel injection port for forming a hollow cone spray. In the present embodiment, the injector 14 is assembled to the cylinder head 4 such that the head portion 14A projects into the combustion chamber 6 at the center in the radial direction B along the cylinder axis direction A. As schematically shown in FIG. 2, the injected fuel F is injected by the head portion 14 </ b> A toward the center region of the combustion chamber 6 in the radial direction B, that is, toward the cavity 5 </ b> C of the piston 5.

  The cylinder head 4 is provided with an intake-side valve operating mechanism 15 and an exhaust-side valve operating mechanism 16 that respectively drive the intake valve 11 and the exhaust valve 12 (FIG. 1). The intake valves 11 and the exhaust valves 12 are driven by the valve mechanisms 15 and 16 in conjunction with the rotation of the crankshaft 7. By driving the intake valve 11 and the exhaust valve 12, the valve element of the intake valve 11 opens and closes the opening of the intake port 9, and the valve element of the exhaust valve 12 opens and closes the opening of the exhaust port 10.

  The intake passage 20 is provided with an air cleaner 21 for purifying the intake air and a throttle valve 22 for opening and closing the intake passage 20 in order from the upstream side of the intake flow. In the present embodiment, during operation of the engine, the throttle valve 22 is basically kept fully open or at an opening close to this. Only under limited operating conditions such as when the engine is stopped, the throttle valve 22 is closed to shut off the intake passage 20.

  A purification device 31 for purifying exhaust gas is provided in the exhaust passage 30. The purifying device 31 includes, for example, a three-way catalyst.

  The EGR device 40 has an EGR passage 41 that communicates a portion of the intake passage 20 downstream of the throttle valve 22 and a portion of the exhaust passage 30 upstream of the purification device 31. Further, the EGR device 40 includes an EGR valve 42 for opening and closing the EGR passage 41, and an EGR cooler 43 for cooling EGR gas passing through the EGR passage 41. The EGR gas recirculated through the EGR passage 41 is cooled by the EGR cooler 43 before going to the intake passage 20.

[Control configuration]
FIG. 3 is a block diagram showing a control configuration of the engine system. The engine system of the present embodiment is totally controlled by a PCM (power train control module) 100. The PCM 100 is a microprocessor including a CPU, a ROM, a RAM, and the like.

  The vehicle on which the engine system is mounted is provided with various sensors, and the PCM 100 is electrically connected to these sensors. For example, the cylinder block 3 is provided with a crank angle sensor SN1 for detecting the engine speed. Further, an air flow sensor SN2 for detecting an amount of air taken into each cylinder 2 through the intake passage 20 is provided. Further, the vehicle is provided with an accelerator opening sensor SN3 for detecting an opening of an accelerator pedal (accelerator opening) (not shown) operated by the driver.

  The PCM 100 executes various calculations based on input signals from these sensors SN1 to SN3 and other sensors to control each part of the engine including the spark plug 13, the injector 14, the throttle valve 22, and the EGR valve 42.

  The PCM 100 functionally includes an ignition control unit 101. The ignition control unit 101 controls the ignition timing of the ignition plug 13, that is, the timing at which the ignition unit 13A forcibly ignites the air-fuel mixture. The engine body 1 of the present embodiment performs premixed compression ignition combustion in which fuel including gasoline is mixed with air and self-ignites, and employs a combustion method in which the ignition plug 13 assists the ignition of the mixture. . That is, the air-fuel mixture in the combustion chamber 6 is forcibly ignited to perform combustion by flame propagation (SI combustion), and the unburned air-fuel mixture in the combustion chamber 6 is self-ignited by the heat generated by the SI combustion (CI). Combustion) (SI + CI combustion) is performed. The ignition control unit 101 controls the timing of ignition assist by the ignition plug 13.

  FIG. 4 is a time chart showing the relationship between the timing of fuel injection and forced ignition in the engine body 1 and the heat release rate dQ. FIG. 4 shows an example in which the fuel injection F1 from the injector 14 is performed in the middle stage of the compression stroke. The fuel injection timing and the injection amount (the lift amount of the injector 14) may be changed according to the engine speed, the engine load, and the like. For example, the fuel injection may be performed a plurality of times in the compression stroke, or a part of the fuel injection may be performed in the intake stroke.

  The ignition control unit 101 controls the ignition plug 13 so that the air-fuel mixture in the combustion chamber 6 is forcibly ignited at a timing before the compression top dead center (TDC). FIG. 4 shows an example in which forced ignition (ignition assist) is executed at a timing slightly advanced from TDC. The ignition assist may not be executed when the engine body 1 is in a specific operating condition, for example, when the engine body 1 is in a low-rotation low-load operation region.

  FIG. 5A is a cross-sectional view showing a state of formation of an air-fuel mixture in the combustion chamber 6. As described above, the injector 14 injects fuel toward the cavity 5C of the piston 5. The PCM 100 sets the injection pressure (lift amount) of the injection to such an intensity that the penetration of the fuel spray from the head portion 14A does not reach the bottom of the cavity 5C. In FIG. 5A, for comparison with the arrangement of the spark plug 13 in the present embodiment (FIGS. 6 and 7) described below, the spark plug 13 The example shown in FIG.

  Thereby, as shown in FIG. 5A, the air-fuel mixture is formed only in the cavity 5C (at the center of the combustion chamber 6). Therefore, this limited gas-fuel mixture layer is surrounded by an air layer containing new intake air, and a so-called stratification of the fuel distribution is achieved. By such stratification, an air layer is formed between the air-fuel mixture layer and the wall surface (combustion chamber wall surface) of the cavity 5C. The formation of such an air layer leads to a reduction in the area where the combustion gas of the air-fuel mixture comes into direct contact with the wall surface of the combustion chamber, so that the cooling loss can be reduced.

  FIG. 5B is a plan view showing an aspect of SI + CI combustion in the combustion chamber 6. Here, a basic combustion mode is described, which does not consider the influence of in-cylinder flow (tumble flow and squish flow) described later. In the combustion chamber 6, SI combustion occurs at an early stage of combustion, and CI combustion occurs at a later stage of combustion. SI combustion occurs in the central region R1 (the region where the cavity 5C is formed) of the combustion chamber 6, and CI combustion occurs in the outer peripheral region R2 around the central region R1.

  That is, as shown in FIG. 5A, the air-fuel mixture is limitedly formed in the cavity 5C. By generating a discharge at the ignition portion 13A of the ignition plug 13 exposed to the air-fuel mixture, the air-fuel mixture around the ignition portion 13A is forcibly ignited. Starting from this ignition point, combustion due to flame propagation occurs in the cavity 5C (SI combustion). If there is no influence of the in-cylinder flow, the flame spreads uniformly in all directions in the combustion chamber 6, and the central region R1 burns by SI combustion. The temperature inside the combustion chamber 6 is increased by the heat generated by the SI combustion, and the outer peripheral region R2 is burned by self-ignition of the unburned air-fuel mixture (CI combustion).

  The above is the basic operation of the SI + CI combustion. However, the flame propagation in the SI combustion may not uniformly spread in all directions of the combustion chamber 6 due to the in-cylinder flow in the combustion chamber 6. That is, as shown in FIG. 5A, even when the air-fuel mixture is stratified in the cavity 5C, the propagation direction of the flame may be unevenly distributed due to the in-cylinder flow. In this case, the region in which self-ignition based on CI combustion occurs in the combustion chamber 6 increases, and the amount of heat generated by the self-ignition becomes excessive, resulting in a problem that combustion noise increases or abnormal combustion occurs. Occurs. In the present embodiment, in order to solve this problem, the location of the ignition portion 13A in the combustion chamber 6 is devised. Hereinafter, this point will be described in detail.

[About the position of the ignition part]
FIG. 6 is a plan view of the crown surface 50 of the piston 5 as viewed from above, and is a diagram for explaining the layout of the ignition portion 13A. In the combustion chamber 6 of the present embodiment, as shown by a dotted line in FIG. 6, two intake ports 9 and two exhaust ports 10 are respectively opened in the combustion chamber ceiling surface 6U. Specifically, two intake ports 9A and 9B are provided on the right side of FIG. 6 in the direction of the ridge line with respect to the ridge line (the portion facing the ridge line portion 54 of the crown surface 50) of the pent roof type combustion chamber ceiling surface 6U. The two exhaust ports 10A and 10B are arranged on the left side at an interval in the ridge direction. Here, in the combustion chamber 6, the side where the intake ports 9A and 9B are disposed is referred to as an intake side, and the side where the exhaust ports 10A and 10B are disposed is referred to as an exhaust side.

  In the crown surface 50, the cavity 5C is arranged in a central region in the radial direction B, and an outer peripheral edge 5E that defines the outer periphery of the cavity 5C is substantially circular in a top view. Part of the inside of the intake ports 9A and 9B and part of the inside of the exhaust ports 10A and 10B and part of the vicinity of the outer peripheral edge 5E of the cavity 5C overlap in the cylinder axial direction A. The squish generation surface 51 is a plane extending outward from the outer peripheral edge 5E in the radial direction B of the crown surface 50 between the two intake ports 9A and 9B. The circumferential width of the squish generation surface 51 is narrowest in the vicinity of the base 51 </ b> E, which is a portion extending from the outer peripheral edge 5 </ b> E (near where the two intake ports 9 </ b> A, 9 </ b> B come closest), and goes outward in the radial direction B. It has become wider. The reason why the squish generation surface 51 has such a shape is that a valve recess is provided in a region of the crown surface 50 facing the intake ports 9A and 9B. A similar squish generation surface exists between the two exhaust ports 10A and 10B, but is not shown in FIG.

  In the structure of the combustion chamber 6 as described above, the head portion 14A of the injector 14 is arranged near the center in the radial direction B, but the ignition portion 13A of the ignition plug 13 is unevenly arranged on the intake side. . More specifically, in a plan view along the cylinder axis direction A, it is located near the inside of the outer peripheral edge 5E of the cavity 5C and between the two intake ports 9A and 9B. In relation to the squish generation surface 51, the ignition portion 13A is arranged so as to be adjacent to the inside of the base portion 51E in the radial direction B. This arrangement is an arrangement in view of the fact that the base portion 51E serves as a region for ejecting the squish flow Sq described later onto the cavity 5C.

  FIG. 7 is a cross-sectional view of the combustion chamber 6 along the cylinder axis direction A. The cavity 5C has a bottom 52 and a slope 53 rising from the bottom 52 toward the outer peripheral edge 5E. As described above, the squish generation surface 51 is a plane extending outward from the outer peripheral edge 5E in the radial direction B of the crown surface 50. In FIG. 7, the position of the ignition portion 13A of the ignition plug 13 is schematically indicated by a circle. This position corresponds to a position serving as a forced ignition point, that is, a position between the electrodes of the ignition portion 13A where a spark discharge occurs. The ignition portion 13A is disposed adjacent to the inside of the outer peripheral edge 5E and above the extension line 51a of the squish generation surface 51 inward in the radial direction B.

  The arrangement position of the ignition portion 13A shown in FIGS. 6 and 7 is an example, and the ignition portion 13A only needs to be arranged in the merging area M shown in these drawings. The merge area M is an area where the tumble flow Tu flowing along the cavity 5C and the squish flow Sq flowing along the squish generation surface 51 merge in the latter half of the compression stroke. This intends to use the tumble flow Tu and the squish flow Sq to direct the flame propagation generated from the ignition portion 13A located at a position unevenly distributed on the intake side toward the center in the radial direction B of the cavity 5C. It was done.

  The in-cylinder flow generated in the combustion chamber 6 will be described with reference to FIG. FIG. 8A is a cross-sectional view in the cylinder axial direction A of the engine body 1 showing the in-cylinder flow during the intake stroke. When the intake valve 11 starts to open and the piston 5 descends, the intake air rushes into the combustion chamber 6 from the intake port 9. Specifically, intake air flows from the intake port 9 toward a portion of the combustion chamber 6 closer to the exhaust side. As a result, a tumble flow Tu having a rotation axis in a direction orthogonal to the cylinder axis direction A is generated in the combustion chamber 6. The tumble flow Tu flows along the cavity 5C and goes to the combustion chamber ceiling surface 6U.

  FIG. 8B shows the in-cylinder flow in the latter half of the compression stroke. The tumble flow Tu gradually disappears with the rise of the piston 5, but the tumble flow Tu remains in the latter half of the compression stroke. Referring to FIG. 7 as well, this remaining tumble flow Tu flows along the bottom 52 of the cavity 5C, rises at the slope 53, and then along the combustion chamber ceiling surface 6U at the center in the radial direction B of the combustion chamber 6. It becomes a heading flow. In addition, a squish flow Sq is generated on the squish generation surface 51. In the squish flow Sq (forward squish flow), the gas existing between the squish generation surface 51 and the combustion chamber ceiling surface 6U loses its place due to the rise of the piston 5 and blows out from the outside in the radial direction B toward the cavity 5C. Is the flow in a cylinder.

  The merge area M is an area where the tumble flow Tu and the squish flow Sq generated as described above merge. Specifically, as shown by a dashed-dotted line in FIG. 7, the merging region M is closer to the outer peripheral edge 5E of the cavity 5C where the squish generation surface 51 extends in a cross-sectional view along the cylinder axis direction A. This is a region that is radially inside and is adjacent to the upper portion of the slope portion 53. As shown in FIG. 6, the merging region M is a region radially inside the outer peripheral edge 5 </ b> E and between the two exhaust ports 10 </ b> A and 10 </ b> B in a plan view in the cylinder axial direction A. This region is a region where the tumble flow Tu and the squish flow Sq naturally join together in the combustion chamber 6 adopting the intake / exhaust two-valve configuration (four-valve type).

  In the present embodiment, as shown in FIG. 7, the ignition portion 13A of the ignition plug 13 is located within the confluence region M and above the extension line 51a extending the line along the squish generation surface 51 to the cavity 5C. The example which is arrange | positioned in the area | region is shown. This region is a region where the squish flow Sq blows out from the squish generation surface 51 to the cavity 5C in the latter half of the compression stroke, and the tumble flow Tu rises along the slope 53 and collides with the squish flow Sq. Area. Although such an arrangement of the ignition portion 13A is a preferable example, as shown by a dotted line in FIG. 7, in a later stage of the compression stroke, the ignition portion 13A is located below the extension line 51a (that is, in the cavity 5C) and has a slope portion. The ignition portion 13A may be arranged at a position adjacent to the upper portion of the 53 so as not to contact the upper portion.

[Reasons for uneven distribution of ignition parts]
Next, the reason why the ignition portion 13A is arranged at a position unevenly distributed on the intake side will be described. First, a comparative example will be described with reference to FIGS. 9A and 9B. In the comparative example, the ignition section 13AX is arranged near the center of the combustion chamber 6 in the radial direction B. As described above, the tumble flow Tu remains in the combustion chamber 6 in the latter half of the compression stroke, and the squish flow Sq is generated. When the ignition portion 13AX forcibly ignites the air-fuel mixture in a state where such an in-cylinder flow exists, the subsequent flame propagation tends to flow by the in-cylinder flow and progress in a direction biased toward the exhaust side. Comes out.

  That is, even when the air-fuel mixture is homogeneously stratified in the cavity 5C (see FIG. 5A), or even when the air-fuel mixture is uniformly formed in the entire combustion chamber 6, the SI combustion is performed by forced ignition. Occurs in the flame propagation zone R11, which does not spread evenly in all directions of the combustion chamber 6, but is unevenly distributed on the exhaust side. In addition, as a result of flowing to the exhaust side, the flame propagation zone R11 itself becomes narrow. The remainder excluding the flame propagation zone R11 becomes an end gas zone R12 in which CI combustion is caused by the unburned air-fuel mixture. This end gas zone R12 is also eccentric with respect to the radial center of the combustion chamber 6 to form an annular region. Specifically, the end gas zone R12 becomes wider as a result of the flame propagation zone R11 itself becoming narrower, and the end gas zone R12 is biased so that the exhaust side becomes narrower but the intake side becomes wider. In this case, a large amount of heat is generated by self-ignition particularly in the end gas zone R12 on the intake side, and the amount of heat generated by CI combustion tends to be larger than the amount of heat generated by SI combustion. This leads to problems such as deterioration of combustion noise and abnormal combustion.

  FIGS. 10A and 10B are diagrams showing a flame propagation zone R11 and an end gas zone R12 generated in the arrangement of the ignition section 13A in the present embodiment. In the present embodiment, the ignition portion 13A is arranged unevenly on the intake side in the combustion chamber 6. When the ignition portion 13A forcibly ignites the air-fuel mixture, the flame propagation is biased toward the exhaust side due to the in-cylinder flow where the tumble flow Tu and the squish flow Sq are combined, which is the same as the comparative example. However, since the ignition portion 13A is unevenly distributed on the intake side, the flame propagation zone R11 formed as a result of the uneven progress is uniformly spread in all directions of the combustion chamber 6.

  In addition, the point of strong in-cylinder flow where the tumble flow Tu and the squish flow Sq join is the forced ignition position by the ignition portion 13A, so that the progress of the subsequent flame propagation is promoted, and the amount of air-fuel mixture burned by SI combustion Increase. That is, as compared with the flame propagation that is pushed out from the radial center portion of the combustion chamber 6 to the exhaust side as in the comparative example of FIG. 9B, FIG. Thus, the flame propagation zone R11 is formed extensively in the central region of the combustion chamber 6. As a result, the end gas zone R <b> 12 becomes a narrow annular region that is substantially concentric with the radial center of the combustion chamber 6. Therefore, the amount of heat generated by CI combustion in the end gas zone R12 is relatively small, and the amount of heat generated by SI combustion tends to be larger than the amount of heat generated by CI combustion. This contributes to suppression of combustion noise and abnormal combustion.

  FIG. 11 is a diagram illustrating a state of the combustion chamber 6 in a later stage of combustion in the present embodiment. This state is an initial stage of the expansion stroke immediately after the piston 5 starts descending from TDC. At this stage, the air-fuel mixture in the cavity 5C is burned by SI combustion, and a reverse squish flow RSq is generated on the squish generation surface 51 as the piston 5 descends. The unburned air-fuel mixture goes radially outward of the combustion chamber 6 by the reverse squish flow RSq, which leads to promoting CI combustion. Therefore, the CI combustion is completed in a relatively short time, and the combustion period is not prolonged unnecessarily.

  FIG. 12 is a graph showing the heat release rate in the combustion chamber 6 of the comparative example and the present embodiment. In FIG. 12, a line indicated by a symbol L is a limit line L at which the combustion noise becomes noticeable and the occurrence of abnormal combustion becomes noticeable. That is, when the heat release rate dQ / dθ exceeds the limit line L, the combustion noise and abnormal combustion exceed the allowable levels.

  First, the heat release rate curve dQ1 in FIG. 12 shows premature combustion that occurs when the above-described SI-CI combustion is not employed at the time of high load and high speed of the homogeneous charge compression ignition engine. In premature combustion, the heat generation rate (combustion pressure) sharply rises, and the combustion period becomes excessively short. The peak value of dQ / dθ in the heat release rate curve dQ1 greatly exceeds the limit line L.

  Next, the heat release rate curve dQ2 corresponds to the comparative example shown in FIG. At the crank angle CA1, forced ignition is performed by the ignition unit 13AX, and SI combustion by flame propagation starts, and CI combustion starts at the crank angle CA2. As described above, under the influence of the in-cylinder flow, the flame propagation progresses toward the exhaust side, and the flame propagation zone R11 becomes narrow. Therefore, the amount of fuel combusted by the SI combustion is small, and the crank angle CA1 to CA2 Slow combustion occurs during the interval. In CI combustion after the crank angle CA2, the unburned air-fuel mixture burns at once. For this reason, in the heat release rate curve dQ2, although the combustion characteristics have a late-stage center-of-gravity type, the peak value of dQ / dθ exceeds the limit line L.

  On the other hand, the heat release rate curve dQ3 corresponds to the present embodiment shown in FIG. At the crank angle CA1, forced ignition is performed by the ignition unit 13A, and SI combustion due to flame propagation starts, and CI combustion starts at a crank angle CA3 that is more retarded than the crank angle CA2. As described above, since the flame propagation zone R11 is evenly spread in all directions of the combustion chamber 6 by forcibly igniting by the ignition portion 13A unevenly arranged on the intake side, the amount of fuel burned by the SI combustion is large. Become. Therefore, in the heat release rate curve dQ3, the period of SI combustion becomes longer, and as a result, the heat release rate dQ / dθ does not increase sharply, and does not exceed the limit line L between the crank angles CA1 to CA3.

  Further, since the amount of fuel consumed in SI combustion is large, the amount of fuel burned in CI combustion after crank angle CA3 is not large. Therefore, also in the CI combustion, the peak value of dQ / dθ does not exceed the limit line L. Moreover, since CI combustion is promoted by the reverse squish flow RSq, the combustion end point is not so retarded. As described above, the heat release rate curve dQ3 has a late-center-of-gravity combustion characteristic, the peak value of dQ / dθ does not exceed the limit line L, and the combustion period is not prolonged. Therefore, according to the present embodiment, combustion noise and abnormal combustion can be suppressed, and thermal efficiency can be improved.

[Effects]
The premixed compression ignition engine according to the embodiment described above has the following functions and effects. The ignition plug 13 assembled to the cylinder head 4 has an ignition portion 13A arranged unevenly on the intake side of the combustion chamber 6. Then, the ignition control unit 101 controls the ignition plug 13 so that the air-fuel mixture is forcibly ignited at a timing before TDC.

  Even if an air-fuel mixture is formed in the central region of the combustion chamber 6 in the radial direction B, the flame generated by the forced ignition is caused to flow to the exhaust side by the influence of the in-cylinder flow such as the tumble flow Tu and the squish flow Sq. There is a tendency to bias toward the exhaust side. However, in the present embodiment, the position deviated toward the intake side in the combustion chamber 6 is the forced ignition point, and the flame propagation is directed to the central region in the radial direction B of the combustion chamber 6 by carrying the in-cylinder flow from the intake side to the exhaust side. Can be changed. As a result, the bias of the flame propagation zone R11 toward the exhaust side is suppressed, and SI combustion occurs exclusively in the radial center region of the combustion chamber 6. In addition, the amount of fuel consumed in SI combustion increases. Thereafter, CI combustion occurs in the peripheral region of the combustion chamber 6 in the radial direction B, but since the amount of unburned fuel is relatively small, the amount of heat generated by CI combustion does not become excessive. Therefore, it is possible to reduce combustion noise and suppress abnormal combustion.

  Further, the ignition section 13A is arranged in a confluence region M where the tumble flow Tu flowing along the cavity 5C and the squish flow Sq flowing along the squish generation surface 51 merge in the latter half of the compression stroke. Therefore, by using the tumble flow Tu and the squish flow Sq, it is possible to direct the flame propagation to the radial center portion of the cavity 5C in which the air-fuel mixture is stratified. In the initial stage of the expansion stroke, the reverse squish flow RSq generated on the squish generation surface 51 can promote CI combustion in the end gas zone R12 in the radially outer region of the combustion chamber 6.

  Further, the merging region M in which the ignition portion 13A is disposed is radially inside the outer peripheral edge 5E of the cavity 5C serving as the base 51E of the squish generation surface 51 in a cross-sectional view along the cylinder axis direction A, and Is an area adjacent to the upper part of the slope 53. Further, the merging region M is a region between the two intake ports 9A and 9B in a plan view of the combustion chamber 6 in the cylinder axis direction. Since such a joining region M is a region where the tumble flow Tu and the squish flow Sq naturally join together, the flame generated in the ignition portion 13A is put on a strong in-cylinder flow and directed toward the radial center of the combustion chamber 6. Is perfect for

  The fuel injection from the injector 14 is performed such that an air-fuel mixture is formed in the cavity 5C and an air layer is formed between the air-fuel mixture and the bottom 52 (wall surface) of the cavity 5C. Thereby, the air-fuel mixture layer and the air layer surrounding the air-fuel mixture layer are stratified in the cavity 5C, and the cooling loss can be reduced. Then, according to the present embodiment, in a state where stratification of the fuel distribution is achieved in the cavity 5C, flame propagation can be uniformly generated in all directions of the cavity 5C.

[Modification]
Although the embodiment of the present invention has been described above, the present invention is not limited to this, and for example, the following modified embodiment can be adopted.

  (1) In the above embodiment, the combustion chamber 6 in which the cavity 5C is provided in the crown surface 50 of the piston 5 has been exemplified. Instead, a piston 5 without the cavity 5C may be used.

  (2) In the above embodiment, the example in which the ignition portion 13A is arranged in the merging region M where the tumble flow Tu and the squish flow Sq merge, but at least the flame is caused to flow radially inward of the combustion chamber 6 by the tumble flow T. It is sufficient if it is in a mode of facing.

  (3) In the above embodiment, an example of a spark plug that generates a spark discharge between the center electrode 131 and the ground electrode 132 has been described as the spark plug 13. The ignition plug 13 only needs to be provided with a discharge electrode for performing various discharges. For example, a plasma jet plug having a discharge electrode for performing a plasma discharge may be used as the ignition plug 13.

  (4) In the above embodiment, the structure in which the squish generation surface 51 of the piston crown surface 50 and the combustion chamber ceiling surface 6U are parallel is exemplified. These two surfaces need not necessarily be formed in parallel, and the squish generation surface 51 may be inclined at a maximum of ± 5 degrees with respect to the combustion chamber ceiling surface 6U. Within this range, the squish flow Sq can be sufficiently generated. As shown in FIG. 5A, the structure in which the squish generation surface 51 is separated from the combustion chamber ceiling surface 6U toward the cavity 5C (the distance between the two surfaces is slightly widened). It may be.

  (5) In the above-described embodiment, an external valve-type injector having a fuel injection port for forming a hollow cone spray is illustrated as the injector 14. Instead, an injector in which a plurality of fuel injection ports are provided in the head portion 14A of the injector 14 may be used.

1 engine body 2 cylinders (combustion chamber wall)
3 Cylinder block 4 Cylinder head 5 Piston 5C Cavity 5E Outer edge 50 Crown surface (combustion chamber wall surface)
51 Squish generating surface 52 Bottom 53 Slope 6 Combustion chamber 6U Combustion chamber ceiling (combustion chamber wall)
9, 9A, 9B Intake port 10, 10A, 10B Exhaust port 13 Spark plug (forced ignition source)
13A ignition part 14 injector (fuel injection valve)
101 Ignition control unit

Claims (3)

An ignition assist type premixed compression ignition engine using a fuel containing at least gasoline,
A combustion chamber having an intake port and an exhaust port, wherein a tumble flow from the intake port side toward the exhaust port side is generated by intake from the intake port, and a compression-ignited combustion of an air-fuel mixture generated by the supply of the fuel; ,
A fuel injection valve that injects the fuel toward a radially central region of the combustion chamber;
A forced ignition source in which an ignition section is disposed in the combustion chamber,
An ignition control unit that controls ignition timing by the forced ignition source,
A part of the combustion chamber wall surface that defines the combustion chamber is formed by a piston crown surface and a combustion chamber ceiling surface facing the crown surface, and the intake port and the exhaust port are formed on the combustion chamber ceiling surface. It is open,
The crown surface includes a cavity recessed in a central region in a radial direction, and a squish generation surface which is disposed at least radially outside the cavity on the intake side and is substantially parallel to the ceiling surface of the combustion chamber. ,
The ignition portion is a position unevenly located on the intake side when the side on which the intake port is arranged in the combustion chamber is an intake side, and the side on which the exhaust port is arranged is an exhaust side, and It is arranged near the inside of the outer peripheral edge ,
The fuel injection valve injects fuel into the cavity in a middle stage of a compression stroke,
The ignition control unit is at a timing before the compression top dead center after the injection of the fuel, and in a cross-sectional view along the cylinder axis direction, the ignition unit radially moves a line along the squish generation surface in a radial direction. The tumble flow flowing along the cavity and the squish flow flowing along the squish generation surface merge in a position closer to the ceiling of the combustion chamber than the extension line extending inward, and the combustion chamber at the timing when the in-cylinder toward the radial direction center area flow is formed at the position of the ignition portion, to force igniting the air-fuel mixture, ignition assist type of pre, characterized in that for controlling the forced ignition source Mixed compression ignition engine. The premixed compression ignition engine according to claim 1 ,
The ignition portion is disposed in a confluence region where the tumble flow and the squish flow merge in a later stage of the compression stroke,
On the ceiling surface of the combustion chamber, two intake ports and two exhaust ports are respectively opened,
The squish generation surface is a plane extending radially outward of the crown from between the two intake ports,
The premixed compression ignition engine, wherein the merging region is a region inside the outer peripheral edge of the cavity and between the two intake ports in a plan view in the cylinder axis direction of the combustion chamber. The premixed compression ignition engine according to claim 1 or 2 ,
The fuel injection from the fuel injection valve is performed such that a mixture is formed in the cavity and an air layer is formed between the mixture and a wall surface of the cavity. Expression engine.

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